The invention relates more particularly to a device for serial treatment of hollow bodies made of thermoplastic material, in particular preforms made of thermoplastic material, comprising:                At least one individual transport element of a hollow body;        At least one treatment rod that is associated with said transport element, with the treatment rod being mounted to slide axially in relation to the transport element;        Controlled means for moving each treatment rod between a retracted position in which the treatment rod is retracted above the hollow body transported by the transport element and an extended position in which the treatment rod is designed to be inserted axially into the hollow body transported by passing through a neck of said hollow body.        
The treatment devices of this type make it possible in particular to treat preforms that are designed for the manufacture, by blow molding or by stretch blow molding, of containers made of thermoplastic material.
In the description and in the claims, the term “treatment” encompasses both physical intervention operations on the hollow body, such as a sterilization or cooling operation, and control operations, such as an operation for measuring the temperature inside the hollow body.
In this type of device, the transport elements move by traveling one after the other along a closed circuit. The transport elements are carried by, for example, a chain or carrousel.
The hollow bodies are loaded one by one on the device in a loading zone. Then, they are moved by the transport elements along a path. During their transport, the hollow bodies are treated. At the end of the path, the hollow bodies are unloaded from the transport elements one after the other in an unloading zone.
When they are preforms, the hollow bodies continue their movement on other devices in the direction of a shaping station, while the transport elements return to their starting point to pick up other hollow bodies to be treated.
In the treatment devices that are already known, the sliding of the treatment rods is controlled by a cam that is arranged along the path of the hollow body.
The cam sliding means are generally very bulky. To make it possible to reduce the span between two successive transport elements, multiple treatment rods are mounted on the same sliding carriage. Thus, all of the rods that are mounted on said carriage are slid simultaneously in such a way as to treat multiple hollow bodies at the same time.
However, the cam sliding control means do not make it possible to adjust the sliding travel of the treatment rods quickly and easily. It is not possible in particular to adjust individually the sliding travel of each rod based on the size of each hollow body during the operation of the treatment device.
In addition, the fact that multiple rods are controlled simultaneously does not make it possible to treat the hollow bodies over their entire path. Actually, in considering a series of multiple successive hollow bodies designed to be treated by an equal number of treatment rods mounted on a common carriage, the first hollow bodies of the series should first of all execute a “vacuum” movement, i.e., without treatment, until the last hollow body of the series is loaded on the treatment device before the treatment can begin. Likewise, the treatment stops simultaneously for all of the hollow bodies of the series as soon as the first hollow body has reached the unloading zone.
The presence of these “vacuum” movements unnecessarily increases the duration of the presence of each hollow body on the treatment device.
Treatment devices are also known in which the control means of the sliding of rods consist of pneumatic cylinders that operate on an “all-or-nothing” basis. The sliding travel of each treatment rod is adjusted simultaneously for all of the treatment rods of the device by the positioning of a stop element common to all of the cylinders. This device makes it possible to roughly adjust the travel of the treatment rods for different types of hollow bodies.
Nevertheless, the common adjustment of the travel of the treatment rods does not make it possible to obtain a fine adjustment making it possible to adapt specifically the travel of each rod to the actual height of each hollow body. This is important in particular when the treatment device is arranged after a station for heating hollow bodies. Actually, the heating brings about a withdrawal of the thermoplastic material that constitutes the hollow bodies. The effect of this withdrawal over the height of the hollow bodies is on the order of several millimeters. In addition, the withdrawal is separate for each preform, and it is very complicated, and even impossible, to determine the exact height of a preform after the heating thereof.
However, certain treatments require a fine adjustment of the travel of the treatment rods during the operation of the treatment device to be able to adapt it to the height of each preform taken individually. Such an adaptation cannot be achieved by the known treatment devices.